Automatic volume control circuits



April 11, 1939. c. N. KIMBALL AUTOMATIC VOLUME CONTROL CIRCUITS FiledNov 17, 1937 :5 INFINITE +3 IMPEDANCE [EAMPL/F/ER mm J r #5 v M 5 row;

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(i/A6155 N. KIMBALL BY WW ATTORNEY.

Patented Apr. 11, 1939 PATENT OFFICE AUTOMATIC VOLUME CONTROL CIRCUITSCharles N. Kimball, East Orange, N. J assignor to Radio Corporation ofAmerica, a. corporation of Delaware Application November 17, 1937,Serial No. 174,943

13 Claims.

My present invention relates to gain control circuits for radioreceivers, and more particularly to automatic volume control circuitsfor a radio receiving system.

In the past, automatic volume control circuits (AVC) for broadcast radioreceivers, that is, receivers adapted to receive signals in a range of500 to 1500 k. 0., have employed a direct current voltage derived fromreceived signals, and proportional' to the signal carrier amplitude, forreducing the mutual conductance, or gain, of the high frequencyamplifier tubes; the latter usually being of the remote cut-off type.There are various disadvantages in such gain control systems,

and it is often desirable to vary the transmission efiiciency of thesignal transmission system by controlling the signal transfer betweenhigh frequency networks of the receiver.

Now, I have found that the signal transfer between a tuned network of ahigh frequency amplifier circuit and a following tuned network may bereadily varied, in response to changes in signal carrier amplitude, byelectric-ally associating with the preceding one of the tuned networksan impedance which varies in some predetermined manner with the signalcarrier amplitude. For example, the impedance can be made properly tovary with signal amplitude so that the associated tuned network feedssignal energy to the following networks in a manner such that the audiooutput level of the receiver is held constant.

The impedance inverting properties of transmission lines of electricallength, equal to odd multiples of a quarter wave length at the frequencyfor which the line is desired, are well known. By employing a quarterwave length line at the operating I. F. of a superheterodyne radioreceiver, the line could be used as an impedance inversion network; and,hence is useful for controlling the signal transfer between a tunednetwork and. following networks. However, such a line would beimpracticable, since physically it could not be tolerated in a radioreceiver. However, all pass lattice type networks can be designed sothat they act as transmission lines of prescribed electrical length,and, also, have the property of impedance inversion. Such an impedanceinversion network employs inductance and capacity related to theterminating impedance of the network so that, as can be readily shown,the input impedance of the network is inversely proportional to themagnitude of the terminating impedance. By associating such an impedanceinversion network with a tuned network in a radio receiver, and varyingthe terminating impedance in magnitude in response to received carrieramplitude change, it is possible to vary the input impedance of theinversion network in a manner such as to vary the signal transfer fromthe tuned network to the following 5 networks.

Accordingly, it may be stated that it is one of the main objects of mypresent invention to pro vide an automatic volume control circuit for aradio receiver, wherein the control circuit com- 10 prises a networkwhose input impedance is a reciprocal function of the terminatingimpedance magnitude, and which input impedance is electricallyassociated with a tuned network of the receiver; the magnitude of theterminating im- 15 pedance being varied in a sense such that the signaltransfer from the tuned network to the following networks is varied tomaintain the audio output level of the receiver substantially uniformregardless of wide variations is received 20 carrier amplitude at thereceiver signal collector.

Another important object of this invention is to provide a receiverequipped with a high frequency amplifier feeding ademodulator through atuned network and a variable impedance net- 5 work of the type whoseinput impedance is a reciprocal function of the magnitude of theterminating impedance; and the said input impedance being arranged inelectrical association with the aforesaid tuned network, the carrier 30amplitude being employed to determine the effective value of theterminating impedance of the impedance network.

Still other objects of my invention are to improve generally theefi'lciency and reliability of 35 AVG circuits for radio receivers, andmore especially to provide AVC circuits, using impedance inversionnetworks, which are not only reliable in operation, but economicallymanufactured and assembled in radio receivers. 40

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically two circuit organizations whereby my invention may becarried into effect.

In the drawing,

Fig. 1 shows a circuit employing the invention,

Fig. 2 illustrates a modification.

Referring now to the accompanying drawing, and specifically to Fig. 1,there are shown only 5 those networks of a superheterodyne receiverwhich are essential to a proper understanding of this invention. Thenumeral I designates an amplifier tube between whose input electrodesare impressed signal energy at an operating intermediate frequency (I.F.) and the I. F. may have a value chosen from a frequency range ofsubstantially to 450 k. c. It is not believed necessary to describe thenetworks of a superheterodyne receiver, which networks would precede theI. F. amplifier I. It is merely necessary to point out that generallysuch networks would comprise a signal collector feeding a tunable radiofrequency amplifier; and the amplified signals being converted to theoperating I. F. by a converter network which includes first detector andlocal oscillator circuits.

The I. F. energy, regardless of how derived, is impressed upon the I. F.transformer 2, whose primary and secondary circuits are each fixedlyresonated to the operating I. F. It is to be understood that the primarycircuit of transformer 2 may be coupled to the output circuit of thefirst detector, or it may be coupled to the plate circuit of a precedingI. F. amplifier. The cathode circuit of amplifier I includes the usualgrid bias resistor-condenser network 3, and the latter provides themaximum amplification bias for the I. F. amplifier.

The tube I is preferably of a type having a low plate resistance value,and such a tube may be of the triode type. The amplified I. F. signalsare transmitted to the demodulator through an I. F. transformer 4 whoseprimary circuit 5 is fixedly tuned to the I. F., the plate of tube Ibeing connected to a point of desired positive potential through a radiofrequency choke coil 6 having a high impedance. It is desired that theprimary circuit 5 of transformer 4 have a low C ratio.

The secondary circuit 1 of transformer 4 is resonated to the operatingI. F., and its high potential side is connected to the control grid ofthe detector tube 8; the low alternating potential side of the circuit 1is established at ground potential. The grounded terminal is connectedto the cathode of tube 8 through a path which includes a load resistor 9shunted by a condenser I II. The condenser III has a magnitude ofapproximately mmf., and acts as a radio frequency by-pass condenser. Theplate of tube 8 is connected to a point of positive potential, and thecathode side of resistor 9 is connected to the input electrodes of thefirst audio frequency amplifier of the audio frequency network throughan audio frequency coupling condenser II. It is pointed out at this timethat the detector 8 is of the infinite impedance diode type. Such adetector has, also, been referred to as a degenerative plate circuitdetector, because there is a degenerative audio voltage across 9.

Normally, that is in the absence of signals, there is a sufficientvoltage drop across resistor 9 to bias the control grid of tube 8 closeto cutoff. Hence, with increasing signal amplitude, the cathode side ofresistor 9 becomes increasingly positive in direct current voltage. Itis not believed necessary further to describe the functioning of thedetector circuit, since it has been disclosed and claimed by P. O.Farnham in application Serial No. 8,864, filed March 1, 1935. It issufiicient to point out at this time that the voltage across resistor 9bears a substantially linear relation to the signal input voltageimpressed on circuit I.

There is disposed in electrical association with the tuned circuit 5 animpedance inversion network generally designated by the numeral I2.

The network comprises a coil I3 connected in series between circuit 5and the plate of tube I4; and, also, a coil I4 between ground and theoathode of tube I 4'.

The tube I4 functions as the terminating impedance of the network I2;that is, the cathodeplate impedance of tube I4 acts as the terminatingimpedance connected between coils i3 and I4 ofthe network I2. Thecathode of tube I4 is connected to ground by a path including condenserI5 and coil I4. The condenser I5 may have a magnitude of approximately0.01 mf. The plate side of coil I3 is connected to the ground side ofcoil I4 through condenser It, while condenser II connects the oppositeterminal of coil I3 to the junction of coil I4 and condenser I5.

Condenser I5 isolates the cathode of tube I 4 from D. 0. ground. This isnecessary because the cathode is at positive D. C. potential withrespect to ground. C15 is 0.01 mi. for it must have an impedance at I.F. which is much less than the lowest value that the Tp of tube I4 mightattain. If C15 is too small, with a consequent appreciable impedance atI. F, it affects the operation of the impedance inverting network, sinceit is a part of the terminating impedance; hence with r of I 4 equal tozero, the terminating impedance is not zero, but is equal to thereactance of C15.

The cathode of tube I4 is connected to a point I8 on the power supplybleeder resistor P; the point I8 is at a positive potential with respectto the grounded end of the bleeder resistor P. The control grid of tubeI4 is connected to ground through a path which includes lead 20,resistor 2| and load resistor 9. The condenser 22 is connected betweenthe grid side of resistor 2i and ground, and the condenser cooperateswith the resistor 2I to provide an audio frequency filter network whichsuppresses audio pulsations in the direct current voltage supplied tothe grid of tube I4. The tube I4 has a relatively low plate resistancemagnitude at zero bias. The plate resistance of tube I and the tunedimpedance of the primary circuit 5 are of low magnitudes, and preferablyless than 100,000 ohms. Furthermore, the constants of network I2 arechosen so that the Q of the coils I3 and I4 is high.

It is essential that the Q of the coils I 3 and I4 be high because thisquantity defines the limits of input impedance, Z, which can be attainedin the network I2. With Tp of I4 zero, the input impedance to network I2is twice the parallel resonant impedance of coil I3 and condenser Ii.The resonant impedance is equal to the quotient of the square of thecoil reactance (at I. F.) and the coil resistance, or equal to whichequals QwL. The input Z is then ZQwL. Q is defined as the ratio ofreactance of a coil, or a condenser, at a given frequency to the A. C.resistance of the coil, or condenser, at the same frequency. It is ameasure of the power factor of the element, and is thus a figure ofmerit. It determines, among other things, the sharpness of selectivityof a tuned circuit in which the element is used. A high Q coil will havea power factor nearer to zero than will a lower Q coil, and will have ina conjunction with a paralleling condenser, ahigher tuned anti-resonantimpedance at any frequency for which its Q is higher.

It will be observed that the input impedance of network I2 iseffectively in series with the tuned impedance of circuit 5. Asexplained heretofore, it is possible to vary the magnitude of thisseries impedance by varying the magnitude of the impedance between theplate and cathode of tube I4. The magnitude of the terminating impedanceof network I2 is adjusted by varying the control grid of tube l4, andthe latter becomes increasingly less negative in potential as the signal carrier amplitude increases. It will be seen that, in the absence ofsignals, the control grid of tube I4 has a predetermined negativepotential with respect to the cathode of the tube. By choosing thisno-signal bias for tube It to be substantially cut-off, then theimpedance between the cathode and plate of tube I4 is substantiallyinfinite, and, hence, the input impedance of the network I2 will berelatively small in magnitude. The input impedance of network I2 hasbeen shown in dotted lines in the drawing, and is generally denoted bythe symbol Z.

It is not believed necessary to enter upon a theoretical demonstrationthat the magnitude of the input impedance Z is a reciprocal function ofthe magnitude of the terminating impedance of network I2. It issufficient to point out that it can be mathematically demonstrated thatthe magnitude of the impedance Z is equal to the product of the ratio ofthe inductances of network I2 to the capacities thereof, and thereciprocal of the terminating impedance magnitude. The magnitudes of theinductances and capacities of network I2 are chosen so that network I2is resonant to the operating I. F.; hence, the magnitude of the inputimpedance Z is inversely proportional to the magnitude of the impedancebetween the cathode and plate of tube I4. Coils I3, I4 and condensersI1, l6 are so chosen that network I2 is resonant to the I. F. in thefollowing manner:--Coil I3 and condenser I6 are arranged so that, ifthey were in the circuit alone, with the Tp of tube I4 infinite, theywould constitute a low impedance to the I. F., i. e., they would be inseries resonance at the I. F. Coil l3 and condenser ll, if connected inparallel would be anti-resonant (parallel resonant) to the I. F. Thisalso applies to coil I4 and condenser It. With the Tp of tube I4infinite, coil I4 and condenser I I are in series resonance. In parallelwith this arrangement, coil I3 and condenser IIi are also in seriesresonance. Each branch circuit has an impedance at series resonanceequal to the coil resistance. Since the two series resonant circuits arein parallel, the net impedance represented by Z is half the coilresistance of the coils if the latter are identical.

If Tp of tube I4 is zero, coil I3 and condenser I! are in parallelresonance, and, in series with this arrangement, coil I4 and condenserI6 are also in parallel resonance. Hence, the impedance represented by Zis equal to twice the parallel resonant impedance of coil I3 andcondenser I1.

As the signal carrier amplitude increases the cathode side of resistor 9becomes increasingly positive in direct current voltage, and the grid oftube I4 becomes increasingly less negative. Hence, the impedance betweencathode and plate of tube [4' decreases. An increase in the magnitude ofthe series impedance Z results in a decrease of the alternating currentvoltage developed across the tuned impedance of circuit 5. The limit isreached when the terminating impedance of network l2 becomes zerothereby giving the input impedance Z its maximum value,

and thus producing minimum signal transfer between circuit 5 and circuitI. The input impedance Z can thus be made to be very large or Very smallby controlling the bias on the grid of tube It. The voltage divisionbetween the circuit 5 and impedance Z is, therefore, a function of thecarrier amplitude; the tuned impedance of circuit 5, and the plateresistance of tube 5, are made relatively small compared to the maximumimpedance of Z so that the AVG range is wide.

The variation of the cathode to plate impedance of tube I4 is at a ratesuch that the impedance Z is varied to maintain the signal carrieramplitude substantially uniform at the input circuit 1 of the detector8. Of course, the network I2 need not be of the specific type shown inthe drawing; for example, it may be of any other type which has theproperty of having its input impedance varied as a reciprocal of theterminating impedance magnitude. Again, the network I2 can be connectedso that its input impedance is in series with the controlled tunednetwork, but precedes it with respect to the plate of the precedingamplifier tube. In general, it is to be understood that my inventioncovers the electrical association of a network of the type designated bythe numeral I2 with a tuned network in a manner such that the signalvoltage across the tuned network may be varied in response to a changein magnitude of the terminating impedance of network I2.

The arrangement in Fig. 2 differs from that in Fig. 1 in that the tube il employs a cathode load network Lo-Co to produce a low terminatingimpedance for network I2 when the tube I4 is operating above cut-offbias. The condenser 6 isolates the plate of tube I from the cathode oftube It for direct current. The coil Lo and condenser Co are resonant tothe I. F.; they are connected between the cathode of tube I 4' and pointI 8 on the bleeder resistor P. When tube I4 is cut off the value of theimpedance of Lo--Co is QwLo. As soon as the carrier amplitude increases,and the grid of tube I 4 gets less negative, the impedance between thecathode of tube I l and ground is QwLO in parallel with of tube I4. Withexceptionally strong signals it is desired that the plate resistance oftube I4 be very low; this degenerative arrangement for tube It attainsthe latter.

If it is desired to avoid cross-modulation effects on the first R. F.amplifier of the receiver, it may be necessary to use the conventionaltype of AVG circuit to supplement the present AVC arrangement; suchconventional AVC circuit causes the gain of the first R. F. amplifiertube to decrease as the signal carrier amplitude increases.

While I have indicated and described two systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. In combination with a, signal transmission tube provided with a tunedoutput circuit, an automatic signal transmission control circuitcomprising a network provided with a terminating impedance and whoseinput impedance is a reciprocal function of the magnitude of theterminating impedance, said input impedance being electrically connectedwith said tuned output circuit, and means to vary the magnitude of saidterminating impedance thereby to adjust in an inverse manner themagnitude of said input impedance.

2. In a system as defined in claim 1, said network including a tubewhose cathode-to-plate impedance provides said terminating impedance.

3. In a system as defined in claim 1, said network having itsinputimpedance in series relation between said tuned output circuit and apoint of relatively fixed potential, and said varying means beingresponsive to signal amplitude at said tuned output circuit.

4. In a system as defined in claim 1, said network including reactancesof opposite sign and being resonant to the operating frequency of saidtuned output circuit.

5. In combination with a source of electrical waves, a load impedanceconnected to said source, a network electrically connected with saidload impedance, said network being of the type whose input impedancebears a predetermined relation to the magnitude of its terminatingimpedance, said input impedance being electrically connected with saidload impedance in such a manner that a change in the magnitude of saidinput impedance varies the alternating current voltage developed acrosssaid load impedance, and means for varying the magnitude of saidterminating impedance.

6. In a system as defined in claim 5, said terminating impedance beingprovided by the oathode-to-plate impedance of an electron-dischargetube, and said varying means adjusting the magnitude of saidcathode-to-plate impedance in response to variations in wave amplitude.

7. In a radio receiver, a signal transmission tube provided with a tunedoutput circuit feeding a detector, an automatic volume control circuitwhich comprises a network of the type whose input impedance is areciprocal function of its terminating impedance magnitude, said inputimpedance being arranged in electrical connection with said tuned outputcircuit in a manner such that an increase in said input impedancemagnitude causes a decrease in alternating current voltage across saidtuned output circuit, and means responsive to an increase in receivedsignal carrier amplitude for varying said terminating impedancemagnitude in a sense such that said input impedance magnitude is causedto vary in a direction to maintain the carrier amplitude at the detectorinput substantially uniform.

8. In a receiver as defined in claim 7, said network including anelectron discharge tube whose cathode-to-plate impedance provides saidterminating impedance, and said varying means including a connection forvarying the gain of said last named tube.

9. In a receiver as defined in claim '7, said network including a tubehaving a tuned load in its cathode circuit, said load providing saidterminating impedance, and said varying means including a connection forvarying the gain of the last named tube.

0. In combination with a signal transmission tube provided with a tunedoutput circuit, an automatic volume control circuit comprising an allpass lattice type network provided with a terminating impedance, theinput impedance of the network bearing a reciprocal magnitude relationto the terminating impedance, electrical connections between said outputcircuit and the said network, and means responsive to variation in thesignal amplitude for adjusting the magnitude of the terminatingimpedance.

11. In combination with a signal transmission tube provided with a tunedoutput circuit, an automatic volume control circuit comprising an allpass lattice type network provided with a terminating impedance, theinput impedance of the network bearing a reciprocal magnitude relationto the terminating impedance, electrical connections between said outputcircuit and the said network, means responsive to variation in thesignal amplitude for adjusting the magnitude of the terminatingimpedance, said lattice network being resonant to the operatingfrequency of the output circuit, and said terminating impedanceincluding an electron discharge tube.

12. In a wave transmission system, at least one resonant circuit tunedto an operating wave frequency, a network comprising inductance andcapacity in predetermined relation and a terminating impedance, thenetwork input impedance bearing an inverse magnitude relation to theterminating impedance, said network being connected to said resonantcircuit and having its input impedance in circuit with the latter, andmeans, responsive to amplitude variation of transmitted waves, forcontrolling the magnitude of said terminating impedance.

13. In a wave transmission system, at least one resonant circuit tunedto an operating wave frequency, a network comprising inductance andcapacity in predetermined relation and a terminating impedance, thenetwork input impedance bearing an inverse magnitude relation to theterminating impedance, said network being connected to said resonantcircuit and having its input impedance in circuit with the latter,means, responsive to amplitude variation of transmitted waves, forcontrolling the magnitude of said terminating impedance, said networkbeing resonant to said wave frequency, and said terminating impedancecomprising a network tuned to said wave frequency.

CHARLES N. KINEBALL.

